1. Field of the Present Invention
The present invention relates to adhesive compositions, and more particularly, to an adhesive composition for use in bonding of a polarizing plate.
2. Description of Prior Art
In recent years, as liquid crystal panels are becoming larger, the upper and lower surfaces of a polyvinyl alcohol (PVA) polarizing plate for use with liquid crystal components are bonded to a triacetate (TAC) protective film to form a three-layer structure. However, the polarizing plate usually operates in a high-temperature, high-humidity atmosphere which predisposes the polarizing plate to expansion and contraction in size, and thus an adhesive for use in bonding of the polarizing plate is susceptible to foaming, peeling, or light leakage.
To improve on the characteristics of the adhesive for use in bonding of the polarizing plate and prevent the adhesive in use in a high-temperature high-humidity atmosphere from foaming, peeling, or light leakage, problem-coping methods adopted are as follows:
1. The weight-average molecular weight of the adhesive is increased such that the weight-average molecular weight of the adhesive controllably ranges between 800,000 and 1,600,000, so as to increase the density of the cross-linking between the adhesive and a polarizing plate and improve the durability of the adhesive, as disclosed in US20090270557. However, increasing the weight-average molecular weight of an adhesive to above 1,200,000 not only increases the viscosity of the adhesive and results in uneven coating but also decreases adhesion. After being bonded to the polarizing plate, the adhesive is unfit to survive dimensional changes in a polarizing plate in a high-temperature high-humidity atmosphere, not to mention that the polarizing plate demonstrates uneven distribution of residual stress as a result of the adhesive thereon, and in consequence the polarizing plate readily comes off from a liquid crystal panel to thereby cause light leakage and other problems with durability.
Conversely, where the adhesive has a weight-average molecular weight of less than 1,000,000 and is bonded to a polarizing plate, the adhesive predisposes an adhesive layer of the polarizing plate to drawbacks arising from poor durability, such as foaming and peeling.
2. A high-molecule adhesive (with a weight-average molecular weight that ranges between 1,100,000 and 1,600,000) is mixed with a low-molecule adhesive (with a weight-average molecular weight that ranges between 30,000 and 50,000) to compensate for light leakage caused by low fluidity of the high-molecule adhesive, as disclosed in JP10-279907. However, the above disclosure in the prior art significantly widens the distribution of the overall molecular weight of the adhesive and thus fails to solve the problems with the durability of the adhesive.
3. A silane coupling agent is added to an adhesive so as to enhance wetting cohesion between the adhesive and an object coated therewith. In so doing, although the durability of the adhesive is slightly improved, the silane coupling agent is extrinsic to the adhesive and thereby does not directly react with acrylic-based copolymer. Also, the silane coupling agent used is greasy silicone of low surface tension and thus, given time, is readily separable from the adhesive and migrates to the surface of the adhesive. Despite the silane coupling agent used, the adhesive intended for use in bonding of a polarizing plate is not effective in bonding the polarizing plate and a liquid crystal panel, as disclosed in US20090270557, and thus fails to meet the demand for reliability of products.